Jury out on PM fenders

PM type (parallel motion) fender systems are seen by advocates as reducing pressure on hulls and the supporting structures, remaining vertical regardless of impact angle and coping with large berthing angles.

However, several installations have suffered structural failures, damaging their reputation. Some of the problems have included water filling of cone fenders and hydraulic locking, failure of low-strength bearing materials, failure of seals allowing ingress of water and debris into bearings, higher internal friction of bearings, seizing of bearings due to loss of lubrication and inadequate software to analyse torsion arm/tube connections properly.

One source has told PS that the principal cause of such failures has been under-design, with designers failing to appreciate the complexity of the forces acting within the PM mechanism.
“PM fenders are less tolerant to abuse than conventional fender systems and, if they are damaged, their complexity makes them more expensive to repair. There are situations where PM fenders are the correct solution but, through effective marketing, they have been installed in a number of locations where a simpler, cheaper and more robust fender would have been appropriate.”

Another consultant summarises PM systems as a brilliant engineering concept not borne out in practice.

“PMs do not remain vertical. Vessels will apply loads parallel to the berth line, either horizontally when moving into position along the berth or whilst ranging in the tide and vertically due to tidal variations.

“The mechanism which supports the fender panel has no means of absorbing any energy in the vertical direction and none in the horizontal direction, when struck from a series of foreseeable angles. Therefore, a ship with an average bow flare which makes first contact near the bow will contact the fender with the bow flare, imposing a substantial vertical load in the process. The PM fender is not designed for this.”

However, Mike Harrison of QuayQuip says his company sees a very bright future for PM systems, although he confirms that highly experienced structural engineers are needed to analyse and design them. PM systems need non-linear design tools and techniques, so manual calculation is impossible, and this cannot be entrusted to inexperienced or unqualified persons.

“The latest QuayQuip PM designs have addressed all of these issues and more,” he says. “We adopt bearing technology well proven for reliability in the dredging industry, including bronze bearings with graphite inserts. We use mechanical engineering load factors in our bearings, some 25% higher than structural steel factors. We also adopt high-strength steel casting for the hinge brackets, which allows complex homogeneous shapes with improved load distribution paths.

“Fatigue is considered on every QuayQuip PM design, along with full finite element analysis. This results in a very high proportion of full penetration butt welds that add to the safety, strength and durability.”

This has led QuayQuip recently to complete the world’s largest contract for PM fenders, for 120 systems at Belvedere, in the UK, worth over €6m ($7.6m). In the past six months, QuayQuip has taken orders worth another €6m for PM systems – some 45 large units in four different contracts around the globe,